Abstract

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The brightness distributions across five intense radio sources have been investigated in the hydrogen line absorption features. In some cases there has been found a variation of the absorption across the face of a source, which, with the kinematical distance to the cloud in question, allowed a typical length to be estimated, which in turn allows estimations of densities and masses. Densities range from a few atoms per cubic centimeter, to, in the case of a small cloud in front of Orion A, as much as 700 atoms per cubic centimeter. Masses range from a few solar masses up to perhaps a thousand solar masses in the cloud causing the deep line in Cas A.

The absorption profiles of the bright sources were analysed into Gaussian components, allowing parameters to be quoted for individual clouds, and, in a few cases, allowing an interesting upper limit to the temperature to be set from the dispersions of the separated components.

The absorption features in several other sources were briefly investigated, including three absorption spectra never before published, and new upper limits for the absorption of apparently unabsorbed sources were set in six cases. This material is briefly analysed statistically, from which it is concluded that the distribution law for central optical depths is approximately [...] and that a line of sight, on the average, intersects 4.1 clouds per kiloparsec in the galactic plane. There is some evidence that the density is about twice this in the local spiral arm.

Some evidence is presented in favor of a "raisin pudding" model of the interstellar medium, in which clouds with temperatures of less than a hundred degrees supply all absorption, and most of the hydrogen line emission, while an appreciable part of the emission profiles is due to a hot intercloud medium of neutral hydrogen, at a temperature of several thousand degrees, and a density of about one half atom per cubic centimeter.